Fresh water goes from the ocean to the air and back to the ocean. The illusion of certainty can be given by this constant shuttling. The tap will always have fresh water. Will it happen?
That is not guaranteed. Climate change is shifting where the water cycle deposits water on land, with drier areas and even wet areas becoming even more so.
The water cycle is changing faster than we had thought, based on changes in our oceans, according to research published today in Nature.
The need to end the emissions of gases warming the atmosphere before the water cycle changes beyond recognition is more pressing than ever.
If this sounds serious, it is. Modern society can be made possible by our ability to harness fresh water.
The water cycle has begun to intensify as the Earth warms up.
More and more freshwater is leaving dry regions of the planet and ending up in wet regions.
What might this look like? The weather intensified. In areas that are relatively dry, there are more intense droughts. Extreme storms and flooding can be found in relative wet areas.
Think of the megadrought afflicting America's west, of the unprecedented floods in Germany, or of the increase in severe rainfall seen in cities like Mumbai.
The shift is already happening. Climate change was already causing long-term changes to the water cycle, according to the UN's Intergovernmental Panel on Climate Change.
The changes we are seeing are just the beginning. This water cycle could make it harder for people to get reliable supplies of fresh water across large areas of the planet over the next few decades.
While we know the water cycle is intensifying, we don't know how much and how fast. The ocean comes into play there.
The main reason it is hard to measure changes to the water cycle is that we don't have enough data.
It is very difficult to set up permanent rain gauge or evaporation pans on the 70% of the planet that is covered in water. When we look at change over the long term, we need to know from decades ago.
Scientists have found a solution that uses the ocean. The ocean can be salty or not depending on the region. The Atlantic is more salty than the Pacific is.
Why? There is rain. When rain falls on the ocean, it makes the sea water less salty. Salt is left behind when water is removed from the surface.
We can use the better- recorded changes in the ocean to detect water cycle changes.
This method was used to track the ocean's surface changes. The research suggested the water cycle is intensifying.
The ocean does not stay like a rain gauge. The ocean's waters are kept in motion by currents, waves, and eddy currents. The link between water cycle change and salinity is uncertain because of this uncertainty.
We have developed new methods that allow us to link changes in the ocean's salinity to changes in the part of the water cycle that moves fresh water.
Our estimates show how the water cycle is changing.
What did we find in the new study? Since 1970, the fresh water equivalent of the waters of the Harbour has shifted from the tropics to the cooler areas. It is estimated that there is between 46,000 and 77,000 liters of water.
This is consistent with an increase in the water cycle. It means up to 7 percent more rain in the areas that are wet and 7 percent less in the areas that are dry.
This is at the upper end of previous estimates, which suggested an intensification closer to 2 percent.
Potentially disastrous changes to the water cycle may be approaching faster than previously thought.
If our water cycle is getting more intense at a faster rate, that means stronger and more frequent extreme weather events.
Even if the world's governments meet their target and keep global warming to a ceiling of 2 degrees Centigrade, the IPCC predicts we would still endure extreme events an average of 14 percent stronger relative to a baseline period of 1850-1900.
The report made clear that some people will be hit harder than others. Mediterranean nations, south-west and south-east Australia, and central America will all become drier, while monsoon regions and the poles will become wetter.
We can expect to see real threats to the viability of cities in dry areas if alternatives are not put in place.
What should we do? You know the answer.
The relationship between greenhouse gas emissions and rising global temperatures has been shown in decades of scientific research.
We must move as quickly as possible towards net-zero emissions to reduce the damage from climate change.
The changes to the water cycle we observed were caused by older emissions. Since then, we have increased our emissions.
It's up to us what comes next.
Taimoor and Jan are associate professors at the University of New South Wales.
This article is free to use under a Creative Commons license. The original article is worth a read.
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